image: fix jffs2(_nand) image generation

image: fix jffs2(_nand) image generation

Variables dependend on JFFS2_BLOCKSIZE and NANDBLOCK_SIZE are used
for template generation, so need to be present before inclusion of
image.mk in target image Makefiles.
So move all declarations to before any includes.

Fixes: r42878 ("image.mk: clean up and parallelize mkfs calls")
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
SVN-Revision: 46564